HNC Electrical Systems Protection - TMA5 Earthing

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MODULE TITLE : ELECTRICAL SYSTEM PROTECTION

TOPIC TITLE : EARTHING

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ESP - 5 - TMA (v1)

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Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

Tees Valley, UK

TS1 3BA

+44 (0)1642 342740

retrieval system, or transmitted, in any form or by any means, electronic, mechanical,

photocopying, recording or otherwise without the prior permission

of the Copyright owner.

This book is sold subject to the condition that it shall not, by way of trade or

otherwise, be lent, re-sold, hired out or otherwise circulated without the publisher's

prior consent in any form of binding or cover other than that in which it is

published and without a similar condition including this

condition being imposed on the subsequent purchaser.

IMPORTANT

Before you start please read the following instructions carefully.

1. This assignment forms part of the formal assessment for this module. If

you fail to reach the required standard for the assignment then you will be

allowed to resubmit but a resubmission will only be eligible for a Pass

grade, not a Merit or Distinction.

You should therefore not submit the assignment until you are reasonably

sure that you have completed it successfully. Seek your tutor's advice if

unsure.

2. Ensure that you indicate the number of the question you are answering.

3. Make a copy of your answers before submitting the assignment.

4. Complete all details on the front page of this TMA and return it with

the completed assignment including supporting calculations where

appropriate. The preferred submission is via your TUOL(E) Blackboard

account:

https://eat.tees.ac.uk

5. Your tutor’s comments on the assignment will be posted on Blackboard.

Teesside University Open Learning (Engineering)

1. Determine the voltage drop at the motor frame for an earth fault at the

motor in the circuit shown in FIGURE 1. The phase voltage is 240 V.

FIG. 1

240 V

Motor

Fault

Motor control centre

Switchgear

Transformer

Earth continuity conductor

0.01Ω

0.0002Ω 0.0002Ω

TXr winding

Teesside University Open Learning (Engineering)

2. An 11 kV motor is fed by cables from a transformer via switchgear,

having a phase impedance of 0.3 + j0.3 ohm. The earth return path to

the transformer neutral has a resistance of 0.42 ohm. Determine a

suitable value of neutral resistance if the voltage rise at the motor in the

event of an earth fault at the motor is not to exceed 430 V. Neglect

transformer winding resistance.

3. Using TABLE 1, determine the resistance of a conductor and the volt

drop per ampere metre at 70°C for a 95 mm2 pvc/swa/pvc copper cable.

The temperature coefficient of resistance is 0.004 per °C.

TABLE 1 Resistances of Copper Conductor and Steel Wire Armour for

3-core copper PVC Insulated Cables at 20°C.

Copper Conductor

Size

Resistance/metre of Conductor

(microhms per metre)

Resistance/metre of Steel Armour

(microhms per metre)

6 mm2

10 mm2

16 mm2

25 mm2

35 mm2

50 mm2

70 mm2

95 mm2

120 mm2

150 mm2

185 mm2

240 mm2

300 mm2

3080

1830

1150

727

524

387

268

193

153

124

4600

3700

3200

2400

2100

1990

1400

1200

1100

740

680

600

540

Teesside University Open Learning (Engineering)

4. A load, 100 m from a 415 V Supply Authority point is connected by a

35 mm2 pvc/swa/pvc copper cable. The current rating is 119 A and the

volt drop per ampere metre is 1.1 mV. If the load current is 90 A,

determine whether the volt drop at the load is within the 17th Edition of

the IEE Wiring Regulations 5% limit. Using TABLE 1, calculate the

earth loop impedance, minimum fault current (neglect the supply/earth

impedance) and select an appropriate fuse for the circuit with reference to

B.S. 88 part 2 (see FIGURE 3, Lesson 4 - 4).

Teesside University Open Learning (Engineering)

Question No.

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